Acetabular components are typically implanted into the acetabular bone in a press fit manner and primarily rely on the ingrowth of bone onto the porous outer surface of the acetabular component. Alternative fixation between the acetabular component and the acetabular bone is possible with multiple different types of screw and peg options. Multiple different types of porous outer surfaces exist to improve the fixation of the acetabular component to the bone as well.
Most acetabular components typically have an opening at their apex (epicenter) of the hemisphere. This opening has a threaded screw hole so that the acetabular component can accept the threads on an acetabular impaction handle. This screw hole is typically located at the apex because the force generated from the mallet impacting the impaction handle is desirably directed in line with the opening of the acetabular component so the component does not change orientation of the opening of the acetabular component. The impaction force is desirably a true compression force that does not create a rotation moment as the acetabular component enters the prepared hemisphere in the acetabular bone. For the same reason that it is beneficial for the impaction force to be directed at the apex of the hemisphere, it is also beneficial for any alternative fixation devices to also direct their force in line with the apex of the hemisphere.
This opening in the acetabular component is sometimes closed with a dome screw cap or cover at the end of the case to prevent the fluid from inside the acetabular component from entering the bone over the 20 to 30 year life span of the implant. Joint fluid can carry polyethylene debris that encourages osteolysis.
Most alternative fixation devices rely on a screw obtaining purchase in the acetabular bone. These screw holes are located at different locations in the acetabular component. The screws are typically located 30 to 40 degrees away from the apex of the acetabular component. The screw holes are located in this position because surgeons typically want to advance screws into a substantial piece of acetabular bone and away from neurovascular structures. The thickness of acetabular bone is greatest in a superior direction, so most screws are positioned in a superior direction. The thickness of the bone located in the direction of the apex of the hemisphere can sometimes limit the amount of bone available to be purchased by a screw. Therefore, screws are often not used in the dome screw hole, but are generally used 30 to 40 degrees away from the apex where the bone is thicker. When the screw is some distance away from the apex, there can be a compression stress between the acetabular bone and acetabular component at the screw location, but a tensile stress on the opposite side of the acetabular component from the moment that is generated by obtaining fixation on just one side of the component. This tensile stress can lead to micro-motion of the acetabular component, failure of acetabular component in-growth and possibly loosening of the acetabular component.
The joint reactive force in an acetabular component is typically directed in a mostly superior and slightly medial direction as the patient's body weight pushes down and the prosthetic hip pushes up. This superior directed force can cause a moment force of the acetabular component that causes a compressive force superiorly and a tensile force medially. The superior directed screw is not ideally positioned to resist this moment force. A medially directed screw would be better positioned to resist this moment force, but the thickness of bone medially prevents adequate screw purchase in most situations.
Alternative fixation between the acetabular component and the acetabular bone is increasingly important in acetabular revision surgery. When a patient has had an acetabular component implanted and then later removed, the acetabular bone is generally less receptive to another acetabular component. Obtaining adequate initial acetabular component fixation is more difficult in a revision setting, and surgeons often have to rely on these alternative fixation devices. Surgeons will often implant multiple screws through the many screw holes in the acetabular component into multiple locations of the acetabular bone.
Various surgical devices in the past have sought to obtain additional fixation between the acetabular component and acetabular bone, but these devices have various limitations. See, for example, the devices disclosed in U.S. Pat. No. 2,765,787 filed on Aug. 2, 1954, U.S. Pat. No. 5,549,691 filed on Feb. 3, 1994, U.S. patent application Ser. No. 20070142921 filed Dec. 21, 2005 and U.S. patent application Ser. No. 20070142922, filed Dec. 21, 2005.